1. Field of the Invention
This invention relates to quality assurance methods and apparatus used for quality assurance of laser surface treatment of a substrate surface and, more particularly, for use of a ballistic momentum apparatus, such as a ballistic pendulum, for quality assurance of laser shock peening.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses one or more radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'd to create the beneficial compressive residual stresses.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following co-pending U.S. patent application Ser. No.: 08/362,362 entitled "ON THE FLY LASER SHOCK PEENING", filed Dec. 22, 1994; and U.S. Pat. Nos.: 5,591,009, entitled "Laser shock peened gas turbine engine fan blade edges"; 5,569,018, entitled "Technique to prevent or divert cracks"; 5,531,570, entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges"; 5,492,447, entitled "Laser shock peened rotor components for turbomachinery"; 5,674,329, entitled "Adhesive tape covered laser shock peening"; and 5,674,328, entitled "Dry tape covered laser shock peening", all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
Laser shock peening processes have been developed to simultaneously LSP pressure and suction sides of leading and trailing edges of fan and compressor airfoils and blades as disclosed in U.S. Pat. No. 5,591,009 entitled "Laser shock peened gas turbine engine fan blade edges" and U.S. Pat. No. 5,531,570 entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges". Single sided shot peened Almen strips are well known for use in the field of shot peening quality control, see U.S. Pat. No. 2,620,838. However, Almen strips do not work with a single laser beam hit. The LSP process involves the use of high pulse energy, short pulse duration laser systems. The combination of high energy and short duration limit the usefulness of electronic measurement systems to verify the true (calibrated) energy being delivered to the component being processed.
Conventional high cycle fatigue (HCF) testing of blades which are LSP'd and notched before testing has been tried as a quality assurance technique. Measurement of the diameter and volume of a single LSP spot on a flat coupon by optical interferometry has also been tried for QA purposes. Both of these methods are fairly expensive and time consuming to carry out and significantly slows production and the process of qualifying LSP'd components. An improved quality assurance apparatus and method of measurement and control of LSP is required which is inexpensive, accurate, repeatable, and quick. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any technique that can reduce the amount or complexity of production machinery and/or production time are highly desirable. According to U.S. Pat. No. 3,469,087, the prior art in the area of measuring the output of lasers includes classical devices such as bolometers, photoelectric sensors, and in one instance, a ballistic pendulum. Ballistic pendulums are well known in the area of physics and in the field of explosive testing as indicated in U.S. Pat. No. 5,728,969. Ballistic pendulums are one form of ballistic momentum apparatus of the present invention which have target masses designed to absorb the force of an impact and explosion, dissipate the imparted momentum, and use the dissipated momentum to indicate the force of the explosion or impact. The ballistic pendulums use gravity to counter and dissipate the momentum such that the momentum of the mass is brought to 0 at the maximum distance of travel of the mass. Linear horizontal ballistic momentum apparatus have horizontally translatable masses that work against a resistance force producing device such as a spring or frictional contact.